Method of drying lumber

ABSTRACT

IN A LUMBER-DRYING METHOD, THE LUMBER IS SUBMERGED IN A BATH OF WATER HAVING A TEMPERATURE OF 80*C. OR HIGHER. WHEN THE LUMBER HAS BEEN SUBMERGED FOR A TIME SUFFICIENT FOR WATER FROMTHE BATH TO BE ABSORBED AND TO AT LEAST PARTIALLY REPLACE THE ORIGINAL COOLOIDAL SUBSTANCES-CONTAINING MOISTURE COMPONENT OF THE LUMBER, THE LATTER IS REMOVED FROM THE BATH AND DRIED BY EXPOSRUE TO HOT GAS, BUT UNDER EXCLUSION OF CONTACT WITH THE AMBIENT ATMOSPHERE, UNTIL THE MOISTURE CONTENT OF THE LUMBER IS REDUCED TO LESS THAN ABOUT 20% OF THE ORIGINAL MOISTURE CONTENT.

United States Patent "ice 3,616,544 METHOD OF DRYING LUMBER Aldo Ottolenghi, 25 de Mayo 294, Buenos Aires, Argentina No Drawing. Filed Sept. 2, 1969, Ser. No. 854,737 Int. Cl. F26b 3/00, 7/00 US. Cl. 349.5 6 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to a novel method of drying lumber.

Known methods of drying and treating lumber include seasoning in the open air and in kilns. When drying in the open air, the timber is generally sawed into lumber or board size pieces which are then stacked, with the interposition of generally short Wooden spacers between each piece in the stacks, so as to prevent warping while ambient air circulates through the spaces between the boards. The moisture in the lumber thus evaporates slowly and drying to the desired moisture content occurs naturally, after a long period of time which may extend to twelve months or more. Drying in lumber kilns generally involves placing the lumber pieces spaced from each other within a kiln having at least one heated air inlet and at least one moist air outlet. The use of kilns shortens, of course, the timerequired for drying. With the aid of modern kilns it is possible to dry one-inch thick boards of yellow pine or gumwood, for example, in about 70 to 80 hours.

In the case of eucalyptus, it has been proposed to submerge the lumber in cold water during 6 to 14 days, then stack the lumber pieces in the open air for about twelve months until the moisture content has been reduced to about 30% and finally stack the lumber pieces in a kiln for about 8 to 12 days.

Drying in the open air has many disadvantages, among which are the following:

(a) It is extremely slow and hence uneconomical, since it immobilizes the timber for a long time and, therefore, the capital represented thereby becomes inactive;

(b) It requires large open spaces for stacking the lumber during the seasoning period;

(c) The dried lumber retains all of the organic substances therein, such substances being food for certain insects which may perforate the lumber and destroy the lumher or reduce the commercial value thereof; and

(d) Valuable substances such as tannin, sugar, starch and others which may be extracted from the wood are lost when drying in the open air.

As mentioned above, the main problem of natural or open air drying is generally removed by drying in kilns through which a current of heated air is forced.

' In both the open air and kiln methods, the elimination of water for converting green or undried lumber into 3,616,544 Patented Nov. 2., 1971 seasoned lumber is efiected from the outside towards the inside, i.e., by drying first the external part in contact with air and then the inner portion which will yield its liquid to the drier outer portions.

When this process takes place slowly, the lumber of boards will not crack. However, when artificial drying is unduly accelerated, the wood in contact with hot air under the action of the heat will dry at a speed faster than can be absorbed by the inner portions of the Wood and will suffer contraction. Superficial cracks will then occur because the inner portions of the lumber will maintain their volume whereas the external portions will contract. This problem has made it necessary heretofore to carry out the drying process in a relatively slow manner.

A further drawback of the kiln drying methods used heretofore is that the liquid in the interior of the lumber, upon migrating under the action of the heat towards the outer portions of the lumber, will carry therewith at least a portion of the soluble substances such as sugar, tannin and starch, and as the moisture is evaporated, these substances will impregnate the outer portions of the lumber and make them harder. This will in turn render the lumber more difficult to work with the ordinary tools, require more power, cause a greater wear of the tools and a lower output of the Wood Working machines.

Drying of lumber is kilns does not permit the extracting of substances such as sugars, starches, etc., which could be used commercially.

SUMMARY OF THE INVENTION All of the drawbacks pointed out above are advantageously overcome by the method of the present invention which makes it possible to dry lumber in a very short time; removes from the lumber at least a major part of the organic substances which cause insects to attack same; avoids the formation of cracks; prevents the formation of a hard external zone in the lumber; permits the recovery of substances such as tannin, sugar, starch, etc. contained in the lumber; and makes it possible to use mechanical means for moving the lumber under process, thus speeding up the steps thereof.

The method of drying lumber in accordance with the present invention comprises the steps of submerging the lumber to be dried in water heated to a temperature of at least about 175 F. C.); maintaining the lumber submerged until the water absorbed by the wood replaces at least in part the liquids containing colloidal substances in the lumber; stopping the submersion of the lumber; subjecting the wet Wood to the action of hot air and removing therefrom, by evaporation, the absorbed Water until the moisture content of the lumber is less than about 20% of the original moisture content.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferably, the water into which the lumber is submerged is heated to a boiling temperature or higher, in which case the internal moisture of the lumber will start evaporating, while the water surrounding it will keep the outer walls of the lumber wet and thus prevent contraction thereof. This brings about a phenomenon which is the opposite of what occurs when drying in the open air or in kilns as heretofore, inasmuch as the lumber starts drying from the inside and not from the outside thereof, and under the action of intense and well controlled heat, the lumber may crack internally due to the rapid evaporation of the internal moisture, which may be useful in making difficult subsequent movements of the dry wood due to variations in the ambient humidity, which in turn causes variations in the humidity of the wood and,

3 therefore, in the size thereof. This result is not obtained with only hot air drying, which would first cause the drying of the outer portions of the lumber, or with vaporization of the wood due to the poor heat conductivity thereof.

Therefore, the method of the invention provides results which neither the hot air nor steam can attain.

The method can be carried out when the water into which the lumber is submerged is below the boiling point, although the drying takes place at a slower rate.

The essence of the method as described above is based on the following facts:

The moisture of the lumber is constituted by liquid which is present in three different forms, namely:

(1) Water as component of parts of the cells and which constitutes about 7% of the total moisture. This is practically impossible to extract since the removal thereof would destroy the cell and, hence, the wood as such. Lumber with 7% moisture is considered to be dry.

(2) Liquid with colloidal substances, namely the liquid including the substances which have been absorbed by osmosis from the membranes of the timber roots, and the substances arriving, transformed, from the leaves. This liquid is the one that more resists extraction, inasmuch as the colloidal solution is of difficult evaporation.

In the case of eucalyptus, natural evaporation of the aforesaid liquid requires up to two years, for boards one inch thick.

(3) Free or intracellular water, which is not found forming colloidal solutions or integrating the walls of the wood cells. This water is relatively easy to extract. Since wood is hygroscopic, there is a certain relationship between ambient humidity and the humidity of the seasoned wood. If during a first part of the method it is possible to substitute a considerable amount of liquid with colloidal solutions by free water, this will facilitate enormously the second part of the drying method, to be carried out with hot air.

The method constituting the main object of the present invention takes into account the characteristics of the different types of liquid or moisture present in green wood.

The difficulty of the artificial or kiln drying methods at present in use is that they try to imitate the natural process, accelerating it without taking into account the possibiliy of using more rapid and economical sysems which are not based on an imitation of spontaneous seasoning. The novelty in the method of the present invention consists in obtaining a more rapid elimination of the moisture.

In accordance with the method of this invention, the natural liquid in the wood is rapidly eliminated, at least in part, and substituted by absorbed water (free water, easy to evaporate) not containing colloidal substances.

Simultaneously, the hot water surrounding the lumber completely, permits the starting of a rapid drying process of the inner portions thereof. While with hot air used in conventional kiln drying methods the lumber becomes relatively little heated, hot water at the same temperature will heat it much more. Wood is a poor heat conductor, but the hot water is a better heat conductor than wood and, as it contacts the liquid contained therein, will rapidly heat such liquid and thus facilitate the rapid elimination thereof by evaporation.

The immersion time will vary according to the type of lumber and the thickness thereof, but as a general rule by heating the water to a temperature between about 100 and 110 C., it will require only about 60 to 80 minutes of immersion for removing, by substitution, at least a part of the original liquids and colloidal substances contained therein, per inch thickness of the lumber.

The absorbed water which has substituted the liquids originally contained in the lumber, is removed in a second part of the method by placing the lumber in a hot air atmosphere which will dry it in only a few hours.

The method as described differs fundamentally from the conventional drying methods in that the latter use only one form of drying which tries to eliminate directly the liquids originally in the wood, whereas the method in accordance with the present invention comprises two basic steps, the first of which is to substitute at least in part the original liquid in the wood by an entirely clean liquid (water) and the second step is to cause the evaporation of the water.

A further advantage of the method of this invention is that it can be carried out with the aid of various mechanical means, for example as follows:

1) By using immersion tanks wherein the water can be heated by heating one or more walls thereof and by the use of hot air, superheated steam, or superheated air, steam or like piping. In accordance with the preferred embodiment, these immersion tanks are constructed below the ground level, since this will facilitate and cheapen the cost of the liquid holding devices. The lumber can be placed within the tank by means of a platform which can be lowered by means of a winch or the like.

(2) By using an endless conveyor, the lumber can be submerged automatically into the hot water and the immersion time can be controlled by giving the conveyor a suitable speed. The same conveyor can remove the lumber from the water tank and carry it to a kiln wherein the hot air will evaporate the absorbed liquid.

It is also possible to use a batch drying method, by draining the water from the tank after an immersion cycle has been completed, and using the same tank for air drying the lumber.

The substances dissolved in the tank water and extracted from the wood as a result of the first step of the method of this invention can be recovered when it is found desirable to do so. These substances can be tannin, sugar, starch, etc., and of these some will settle on the bottom of the tank while others will remain in suspension and can be recovered by evaporating the water containing them or by any other suitable means.

Another advantage of the present method is that it permits the use of trays made of a network of stainless steel or any other suitable rust-proof material for supporting the wood under treatment. In this manner all sides of the lumber can be exposed to the water or air, thus facilitating the action of said fluids.

As mentioned above, artificial drying is generally carried out in kilns through which and by means of fans there is circulated a current or air which is artificially heated and forced into the kiln, through which it will circulate rapidly between the lumber pieces or boards. In contact with the wet surface of the lumber, this air will carry some of the moisture. The humid air is removed and replaced by drier air, and so on. This method is uneconomical for the following reasons:

(a) It requires heating large masses of air constantly renewed, with the corresponding high fuel costs;

(b) It requires a large number of fans, with a resulting high power expense; and

(c) The air forced into the kiln has the ambient humidity and, therefore, a limited capacity of absorbing humidity from the lumber, thereby extending the drying time. The system in accordance with the present invention is simpler, more rapid and cheaper.

Instead of continuously forcing heated air with the ambient percentage of humidity into the kiln, and continuously removing the moisture laden air, in accordance with a preferred embodiment of the invention a closed hot air kiln is used, without heated air inlets or humid air outlets, all of which is substituted by moisture extractors, with the following advantages:

(I) A cheaper kiln construction by avoiding the use of fans and the air inlet and outlet piping;

(II) Reduced operating expenses. It is much cheaper to maintain the heat in a closed and heat insulated atmosphere than in a room into which air is forced which has to be heated; and

(III) Greater drying speed. Theoretically, moisture extractors can reduce to zero percent the humidity of the air, whereas the atmospheric air has a humidity which in some instances can reach 80% or 90%.

It is obvious that in a kiln wherein practically dry air is circulated, the surface moisture of the lumber will be absorbed by said air much more rapidly than in a kiln through which air having a humidity content of 80% or 90% is circulated.

While preferred embodiments of the invention have been described above, it should be understood that the scope of the invention is only limited by that of the appended claims.

I claim:

1. A method of drying lumber, comprising the steps of submerging the lumber to be dried and having an initial moisture content, in a bath of water having a temperature of at least 80 C.; maintaining the lumber submerged in said bath until it absorbs sufficient water from said bath to at least partially replace the colloidal substances-containing initial moisture component of the lum ber; terminating submersion of the lumber in said bath; and subjecting the wet lumber to the action of hot air in a confined space under simultaneous exclusion of the ambient atmosphere, so as to reduce the moisture content of the lumber to less than about 20% of said initial moisture content.

2. A method as defined in claim 1, wherein the temperature of the water in said bath is at least equal to the boiling temperature thereof.

3. A method as defined in claim 1, wherein the replaced colloidal substances-containing initial moisture component enters into said bath; and further comprising the step of recovering said substances from said bath.

4. A method as defined in claim 1, wherein the sRep of submerging the lumber in said bath comprises supporting the lumber in said bath on a plurality of superposed spaced levels.

5. A method as defined in claim 1, wherein the step of submerging the lumber in said bath comprises supporting the lumber within said bath at a plurality of spaced locations so as to facilitate contact of the water with said lumber.

6. A method as defined in claim 1, wherein the steps of reducing the moisture content of the lumber comprises evaporating the moisture in the lumber under the influence of said hot air so as to expel the moisture into said confined space as a vapor, condensing the vapor in said confined space, and extracting the condensate from said confined space.

References Cited UNITED STATES PATENTS 733,8'35 7/1903 Gray 349.5 X 783,868 2/190'5 Jarratt 349.5 1,502,311 7/1924 Mason 34-9.5 1,593,598 7/1926 Redman 34-13.4

JOHN J. CAMBY, Primary Examiner US. Cl. X.R. 34-13.8 

